What is attainable? I asked this question about the self-assembly part of such a system to Jim Kelly of Lego Mindstorms. He replied:

I'd estimate that most robots I've seen include the following electronics:
1. Brick (processor)
2. 3 motors
3. 2-3 sensors

The rest is plastic parts. It's hard to give you a percentage, but I'd say that most robots are 80% non-electronic pieces… you have to have the motors and brick to at least give the robot movement/action.